Domain ranking is frequently used by search engines to facilitate the ordering of results for a search query. In general, a domain may be ranked based in part on the number of contributing domains associated with that domain. A contributing domain may be, for instance, one that includes a link to the domain being ranked. For example, if hundreds of other domains that maintain at least one web site include one or more links to a particular domain, the domain may receive a higher rank than another domain that is referenced by just a few other domains. In addition to the number of contributing domains, ranks of the contributing domains may influence the rank for the domain as well. For example, if a trusted, popular domain, such as Yahoo.com or CNN.com, includes a link to the domain, the ranks for such popular domains may be attributed to the rank for the domain. Receiving a higher domain ranking often means that the domain would be listed above other competing domains, thereby affording more visits by those browsing or searching the Internet. For domains that maintain commercial web sites or web sites that charge advertisers on per-click or per-visit basis, a higher domain ranking means better profitability. For example, sites like Yahoo.com and CNN.com, which are visited by millions of people each day, attract many commercial advertisers who are willing to pay large fees.
Typically, a domain includes links to other domains to make its content more useful and attractive for its visitors. Most existing domain ranking algorithms often assume that a number of contributing domains that maintain one or more links to a particular domain provides an indication of the popularity or utility of the particular domain. Those algorithms also tend to assume that the particular domain is popular and/or useful if a link is included in another domain that is well-known for its popularity and utility, such as MSN.com. These assumptions have been held to be mostly correct when it was neither easy nor cheap to register and maintain a domain.
Due in part to increased competition in the domain registration market in recent years, however, the cost involved in purchasing a domain has decreased significantly. In some cases, domain registrars even offer free domain registrations for up to thirty to sixty day trial periods. Spammers often take advantage of such offers through a spam technique known as a web farm. In particular, spammers purchase or otherwise obtain a large number of sites and interlink the sites together to increase the sites' rankings by artificially increasing the number of contributing domains for some or all of the sites. In effect, this practice defeats the assumption that the more a domain is referenced by other domains, the more likely that the domains is popular and/or useful such that it should be highly ranked.